Yet another bicyclist has died after a crash on a popular Peninsula road.

Deborah Johnson, a 54-year old Palo Alto business consultant, had been riding with a group of friends on Sand Hill Road about 1 p.m. Sunday when she fell and struck her head, according to Menlo Park police.

Police are still investigating the cause of the accident and it is unclear whether a car was involved.

Johnson was taken to Stanford University Medical Center, where she was pronounced dead Tuesday.

Johnson - a longtime biker - is one of several bicyclists killed on the Peninsula in the past year. Last month, Rodney Smith, a Portola Valley philanthropist, was believed to be the fourth cyclist killed in the past year after an elderly driver hit him on Sand Hill Road near I-280.

After the rash of accidents on Sand Hill, San Mateo County is repainting bike lanes and adding signs and considering lowering the 55 mph speed limit. The county recently placed lane dividers on the road. And, one witness told the Mercury News that he saw Johnson strike one of the dividers. They have since been removed.

"It's a terrible shock, riding on a bicycle could be so violent," said Carol Blancher, a friend and a colleague of Johnson's. "She was completely professional, conservative and careful about everything that she does, including riding a bike."